In pumps of the down-locking type, it is desirable to maintain the lower check valve of the pump well seated when the plunger is fully locked down in order to keep the inlet to the pumping chamber securely closed during shipment, storage or other handling. Typically, plungers have utilized hold-down components at their lower ends for engaging and retaining the valves closed at these times.
Products to be dispensed by pumps of this type frequently have several constituents, certain of which may tend to settle out and separate from others when the product is left in a static condition for any given length of time. Yet it is important that the product actually dispensed be as nearly homogeneous as possible so that the user receives a dose or portion representing a true fraction of the product as a whole in its homogeneous state, not just a quantity of the liquid carrier or other individual constituent of the product.